The hypothalamus secretes thyrotropin-releasing hormone in response to nervous stimuli. The TRH then stimulates the pituitary
gland to secrete thyrotropin-secreting hormone (TSH). The follicular cells of the thyroid gland then release thyroid hormone
in response to TSH. Thyroxine (T4) and tri-iodothyronine (T3) provide negative feedback primarily at the level of the pituitary.
Thyroxine is the primary iodothyronine, or thyroid hormone, released from the thyroid gland. Tri-iodothyronine is also released
in a much smaller quantity. Thyroid hormones consist of both free and bound fractions. Ninety-nine percent are bound to albumin,
prealbumin and thyroid hormone-binding globulin. It is the free fraction of T4 that can enter cells where it is converted
to the biologically active portion, T3 as well as rT3.
Total serum thyroxine (TT4) consists of free and unbound fractions and is usually measured with human radioimmunoassay's (RIA)
and ELISA's validated for the dog and cat. There are in-house ELISA's that measure TT4 but there are conflicting results regarding
their accuracy. Anti-thyroxine antibodies can artificially increase TT4 levels with some assays. Non-thyroidal illness and
certain drugs can falsely decrease the TT4. Total thyroxine is commonly used as a screening test for dogs with hypothyroidism
and cats with hyperthyroidism.
Free Total Thyroxine
In both dogs and cats, non-thyroidal illness can falsely decrease TT4 because total serum thyroxine can be affected by the
quantity of carrier proteins, alterations in metabolism, the ability to transport thyroxine into cells and the binding of
T4 within the cells. Anti-thyroxine antibodies occur in about 2% of hypothyroid dogs and can falsely elevate TT4. It is also
possible that cats with early hyperthyroidism may have TT4 levels that wax and wane despite clinical signs and a normal TT4
may be found in a hyperthyroid cat. For these reasons, the unbound or free fraction of TT4 is often measured in dogs and cats
in which TT4 is non-diagnostic. Free T4 is not altered by those factors that affect TT4 in non-thyroidal illness. Tests for
fT4 using equilibrium dialysis (ED) are recommended. Equilibrium dialysis also eliminates anti-thyroxine antibodies. Because
ED is time-consuming and expensive, human radioimmunoassay's (RIA) have been used in dogs and cats. These RIA's consistently
measure a lower fT4 than ED and are of no diagnostic value over TT4. Because fT4 is a more sensitive test in the dog and cat
it seems logical to use it as a screening test but unfortunately a few euthyroid dogs have a low fT4 and some euthyorid cats
have an increased fT4. So although the fT4 is more sensitive it is not a perfect test. Free T4 is commonly used in dogs and
cats when the TT4 is non-diagnostic.
The pituitary secretes thyroid-stimulating hormone (TSH) in response to thyrotropin-releasing hormone (TRH) from the hypothalamus.
Thyroxine provides negative feedback so in dogs with primary hypothyroidism resulting in a low T4, the endogenous TSH should
be increased. Unfortunately the TSH of many hypothyroid dogs is within the reference range. This may be due to fluctuations
in TSH, the effects of drugs or concurrent disease on TSH production and the presence of secondary (pituitary) or tertiary
(hypothalamus) hypothyroidism. The TSH might also be increased in euthyroid dogs which may be due to early hypothyroidism,
recovery from non-thyroidal illness and certain drugs. Endogenous TSH is a less sensitive test than either TT4 or fT4. Endogenous
TSH is most often used concurrently with TT4 and fT4 for diagnosis of hypothyroidism in the dog. Currently there are no assays
available to measure TSH in the cat.
TSH Response Test
Unfortunately, in some dogs it may be difficult to determine if they are hypothyroid with the tests mentioned above. In these
cases a baseline TT4 can be measured and then human recombinant TSH (25 to 100 µg) administered IV and another TT4 measured
at 6 hours. In the presence of a normal thyroid stimulation of TT4 should occur but in primary hypothyroidism little stimulation
should occur and, in fact, both samples may be below the reference range.